Which is better for fuel economy, windows open or AC on?

It’s hot and sticky, but what’s really best? Opening the windows or turning on the air con? It’s a question that many drivers have pondered over the years, with many of you may already be aware there is a magic number to take into account when it comes to deciding which is a more efficient way to keep cool; 45mph.

Below this speed it makes sense to open your windows to let a breeze. Above this speed, the drag effect of the windows being open, begins to make it more fuel efficient to shut them in favour of switching on the air con.

Air Conditioning control panel Citroen C4

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This advice is based on the results of a 2004 study by the Society of Automotive Engineers (SAE), which studied the impact of windows open vs. A/C in large saloons and sports utility vehicles.

Guideline advice

There are plenty of variables that this study couldn’t have accounted for like, how much someone might open their windows, the aerodynamics of different car models, the how long someone might use their air con, as some use might only use it for a short blast and the option of using air vents and fan, without turning on the air con.

As such the 45mph figure is a guide, but a fairly good one. While all this might seem like a minor issue, getting it wrong can have a notable impact on your fuel economy; using AC can increase your fuel consumption by as much as 10%. Opening the windows at higher speeds can increase fuel consumption by as much as 20%.

Audi A3 window controls

But the windows v air con debate isn’t just about working out which is more fuel efficient. It is also worth considering your exposure to air pollution. While the 45mph line is a good guide, things get a bit more complicated when you are in bumper to bumper traffic.

In these situations, it is probably best to avoid using air con or opening your windows. However we know that in hot conditions that might be uncomfortable, so as a solution you could turn on the air con but close your air vents, so the air con merely recycles the cabin air rather than drawing the air from outside, where there is nose-to-tail line of traffic all belching out exhaust fumes.

What’s best for fuel consumption?

There are old-fashioned ways too that you could consider to keep your car cooler too, such as trying to park in the shade or in a garage, opting for tinted windows or heat deflectors or just using a fan to keep cool.

In some cars you can use the AC guilt free; cars like the Toyota Prius and Nissan Leaf, which are available with optional small solar panels on the roof, can power auxiliary functions like the air con from this energy generated from solar, meaning there is no drain on the car’s battery or additional fuel burnt.

petrolfillip

Meanwhile, another study has revealed that hybrid car fuel economy is harder hit by air con use than conventional combustion only cars, which might be a point worth considering if you are one of many hybrid owners who won’t have opted for the solar panel.

In this study by Emissions Analytics, it was found that the impact of AC on hybrid fuel economy was almost twice as much, with fuel efficiency dropping 6.1% in a hybrid car on average, compared to just 3.8% for standard petrol car and 4.6% for a diesel. This might be something worth bearing in mind, if you drive a hybrid.

Do you need to use the AC?

There might be purist among you who think that the very notion of air con in cars in this country is a crime against fuel efficiency but we know that for many Brits, air con is now just a standard feature they expect to find – and be able to use – all we’d say is use it wisely and sparingly and you can stay cool without burning a hole in your wallet.

We could go into the environmental impact of various AC systems and the refrigerant gases they use, but that is probably for another article.

In most driving conditions though, apart from on the motorway, it’s likely that a crack in the window, will work better for you than turning on the AC, but the choice is of course, yours.

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